1. Field of the Invention
The present invention relates to data transmission systems with electromagnetic transponders and, more specifically, to the data transmission from a contactless and wireless electromagnetic transponder to a read/write terminal.
2. Discussion of the Related Art
Electromagnetic transponders to which the present invention more specifically applies are transceivers with no autonomous power supply, which extract the power required by the electronic circuits that they comprise from a high-frequency field radiated by the antenna of the read/write terminal. Such electromagnetic transponders are based on the use of oscillating circuits on the transponder side and on the read/write terminal side. These circuits are coupled by a close magnetic field when the transponder enters the field of the read/write terminal.
The terminal's oscillating circuit is excited by a high-frequency signal (for example, 13.56 MHz) to be sensed by a transponder. When the transponder is in the terminal's field, a high-frequency voltage is generated across the transponder's resonant circuit. This voltage, after rectification, provides the supply voltage of the transponder's electronic circuits which generally comprise a microprocessor, a memory, a demodulator of signals that may be received from the terminal, and a modulator for transmitting information to the terminal. These electronic circuits may also be circuits in wired logic.
The transmission of information from the transponder to the terminal is generally performed by modifying the load of the oscillating circuit of this transponder, so that the latter draws a greater or smaller amount of power from the high-frequency magnetic field. This variation is detected on the terminal side since the amplitude of the high-frequency excitation signal is maintained constant. Accordingly, a variation in the power drawn by the transponder translates as an amplitude and phase variation of the current in the terminal's antenna. This variation is then detected by means of a phase demodulator and/or an amplitude demodulator.
The load variation on the transponder side is generally achieved by means of an electronic switch for controlling a resistor or a capacitor modifying the load of the oscillating circuit. The electronic switch is generally controlled at a so-called back modulation sub-carrier frequency (for example, 847.5 kilohertz) much smaller (generally, with a ratio of at least 10) than the frequency of the of the read/write terminal's oscillating circuit excitation signal.
U.S. Pat. No. 6,356,198, which is incorporated herein by reference, describes an electromagnetic transponder capacitive modulation system. It provides, between the transponder's resonant circuit and its rectifying means, a capacitive modulation circuit intended to modify the transponder load on the field radiated by the terminal.
A recurring problem of transponder systems is that different factors have an influence upon the transmission. Not only is the distance separating the transponder from the terminal likely to modify the amplitude and the phase of a signal received by the terminal, but also will electromagnetic disturbances of the system environment have an influence upon this transmission. The effects of this environment are generally desired to be reduced by maintaining the tuning of the oscillating circuits of the terminal and of the transponder on the same frequency (13.56 MHz). For this purpose, the tuning of the transponder's resonant circuit is generally modified by modifying (for example, by means of switchable capacitors) the equivalent value of the capacitor in parallel on the antenna of this resonant circuit. Such a tuning matching is described, for example, in U.S. Pat. No. 5,892,300, which is incorporated herein by reference. It is not a modulation at the rate of the sub-carrier to transmit information, but simply a modification of the tuning of the resonant circuit. Such a tuning matching especially enables maximizing the power received by the transponder remotely supplied by the terminal. This however does not solve the problem of the demodulation, by the terminal, of information transmitted by the transponder.
Another difficulty lies in the fact that neither the transponder nor the terminal knows, before transmission, whether transmitted data will be properly received.